These days the pollution problem is becoming one of the major concerns in many industries. In the iron and steel industry, it has become necessary to treat the dust discharged from the various types of furnaces in order to reduce or eliminate the environmental pollution caused by such dust. Usually such dust is caught and retained by means of collectors within the plant and then disposed of by one or more ways -- burning, dumping, using as aggregate, etc.
One way of re-utilizing such collected dust is to use it as part of the materials for sintering processes, for example, for producing pellets. However, due to the large volume of dust collected, particularly that part consisting of fine particles, only a part of the dust has been re-utilized to date and most has been dumped. The dumping also creates a pollution problem and, also, the dumping of the reusable materials contained therein constitutes a big waste.
Therefore, in order to reduce pollution as well as to save resources, full-reutilization of the iron and steel industry dust has been considered, and ways of reusing such dust generally fall into two categories, that is:
A. mixing the dust with a suitable binding agent to produce building or constructing materials which are to be used outside the steel industry; and PA1 B. recycling the dust within the steel industry where the dust is produced as a resource for manufacturing iron and/or steel by modifying the composition and shape of the dust.
The present invention is related to recycling the dust as mentioned in "B" above and particularly to the manufacture of pellets comprised mainly of the dust collected by wet type equipment such as venturi-scrubber precipitator or the like.
In the course of producing pellets from the dust collected by the wet type collector, the water is removed from the dust, contained in a slurry state and, after dehydration, the dust is in the state of a cake is which, hereinafter referred to as "filter cake". This filter cake is then fed to the next stages including a drying stage before being fed to a pelletizing apparatus.
If the quantity or moisture content of the filter cake supplied to the drying stage and the stages succeeding thereto varies, such variation influences the next stages remarkably. Therefore, in drying the filter cake, it is necessary to provide a means for controlling heating capacity over a relatively wide range to meet the variation in quantity of the filter cake supplied. Also, if the moisture content of the filter cake is high, large heating capacity is required and this increases the fuel cost.
Thus, there has been a need for a process insuring that the filter cake is supplied to the dryer and the following stages at a constant rate and with the moisture content held within predetermined limits.